The distortion of voltages and currents on utility electrical networks is increasing. The distortion problem is largely attributable to a growing number of nonlinear loads on the utility electrical network. Typical nonlinear loads are computer controlled data processing equipment, numerical controlled machines, variable speed motor drives, robotic devices, electronically switched loads such as lamp dimmers, rectifier loads (especially single phase devices), battery chargers (e.g., battery chargers for electric vehicles), medical equipment, and communication equipment.
Nonlinear loads draw square wave or pulse-like currents instead of purely sinusoidal currents drawn by conventional linear loads. As a result, non-sinusoidal current flows through the predominantly inductive source impedance of the electrical network. Consequently, a nonlinear load causes load harmonics and power to flow back into the power source. This in turn produces unacceptable voltage harmonics.
Harmonics create a variety of problems. For example, harmonic currents cause increased eddy current and hysteresis losses in the iron cores of transformers and other magnetic devices. By producing unwanted noise on communication channels, harmonic currents may interfere with the normal operation of communication circuits. The same problem can occur in power system protective relaying circuits that rely on communications channels. The harmonic problem is compounded by the growing presence of power factor correction capacitors on utility electrical networks. Power factor correction capacitors detrimentally operate as low impedance paths to higher order harmonics. Consequently, harmonics may overload power factor correction capacitors and force them to fail due to increased dielectric losses and stress.
The degree of current or voltage distortion can be expressed in terms of the relative magnitudes of harmonics in the waveforms. Total Harmonic Distortion (THD) is one of the accepted standards for measuring voltage or current quality in the electric power industry.
Although harmonics are present at all levels of a power system, the magnitude of the problem is of main concern in primary and secondary distribution circuits. The primary distribution circuits are the high voltage circuits that are energized by the sub-transmission or transmission systems. Secondary distribution circuits are the lower voltage circuits at the secondaries of distribution transformers. In the primary distribution circuits, the current THD levels are usually low (approximately 5-20%). In the secondary distribution circuits, the THD levels are greater (typically 7-30%).
Harmonic standards have been introduced all over the world. IEEE Standard 519 serves as a guideline for designing systems with linear and nonlinear loads. It also defines the quality of power to be supplied by the utility to the consumer at the point of common coupling. In enforcing the standards, it will be necessary to determine who, the utility or the consumer, is responsible for some or all of the distortion. Consequently, it would be useful to be able to determine within reasonable accuracy, the loads responsible for the generation of harmonics. Thereafter, known filtering techniques and other electronic measures can be used to mitigate the harmonics.
There has been a considerable effort placed on the study of the propagation of harmonic signals from a given source into the network. Most of the work focuses on modifying the power flow algorithm to accommodate harmonic signals. Although generators are the source of all energy on a utility electrical network, harmonic flow studies treat non-linear loads as the source of harmonics in similar fashion that loads are treated as sinks in power flow studies.
Sophisticated work has been performed to simulate the flow of harmonics into the utility electrical network. However, this work requires a priori knowledge of the location of a harmonic source. Furthermore, the measurements need to be highly accurate in both magnitude and phase of harmonic current and voltage.
Prior art techniques for identifying harmonic sources are relatively complicated. For example, one technique requires multiple measurements at various buses within a network to identify a harmonic source. Another technique is to gather information at a single location and subsequently use Fourier transforms to find the magnitude and phase of the harmonic components in the current and voltage waveforms. Because the interpretation of this data is difficult, artificial neural networks have been proposed to simplify the task. Unfortunately, artificial neural networks add another computation to the process of identifying a harmonic source. Moreover, a time-consuming training operation must be performed before the neural networks can be used.
Thus, it would be highly desirable to provide a relatively simple mechanism to identify the presence of a harmonic producing load on a utility electrical network.